In rev. 10303 guidelines shift+ctrl rotation has been changed to consider the guide initial angle as base. The absolute rotation was made by purpose, it was not a bug to be fixed. Guidelines are absolute objects and it was decided to make them snap at absolute angles, regardless of their initial orientation. Also, now a relative rotation is made possible by the new math in input fields feature: just open a guide, then write a +15 in the angle field and it's done.
Personally I find the "relative" option of little usefullness. Should I need it, there is already a simple way to obtain it, while I use a lot absolute snapping so I can "recycle" guides already on screen rather than throwing a lot, having to clean them one by one.
More: the math needed requires trigonometric functions and this makes me think that repeatedly rotating a guide will inevitably lead to rounding errors so you could eventually end up with a guide not starting from where it was at the beginning. This is a huge problem for technical drawings and would force me to carefully avoid using this function (this means always opening the guide menu and input the desired angle or always start from a vertical or horizontal guide and make a single rotation!).
I ask for reverting back to the previous behaviour, please. Or add a different modifier for the two options (but we already know that there are less modifiers than the functions involved with guides...). Regards. Luca